During tumor formation subsets of tumor cells demonstrate dynamic and reversible phenotypes that result from shifts in epigenetic regulation of the genome. This epigenetic plasticity is hypothesized to contribute to quick acquisition of chemotherapy resistance and encourage tumor-initiating potential. One such epigenetic feature found in aggressive cancers is the presence of bivalent promoters. Bivalent promoters are characterized by paradoxical active H3K4me3 and repressive H3K27me3 histone marks. The role of promoter bivalency in mediating increased survivability during tumor recurrence remains mysterious and may provide new therapeutic targets in preventing or treating recurrent breast cancer. Using a mouse model that emulates breast cancer recurrence, we have found that repression of tumor suppressor par-4 is critical for primary tumor survival and accelerated tumor recurrence. Furthermore, we have determined par-4 is transcriptionally repressed by the presence of bivalent histone marks. Par-4 repression is reversible with HDAC inhibitors and its re-expression sensitizes recurrent tumor cells to conventional chemotherapies. Aim 1 of this proposal will investigate the mechanism of par-4 chemosensitization, and whether the effect is translated to our GEM model. Our previous data with par-4 suggests that reactivation of bivalent promoters may have functional significance in regulating therapeutic resistance and recurrence. We?ve identified 564 genes through ChIP-Seq analysis that are regulated by promoter bivalency. In Aim 2, I will evaluate the functional significance of these bivalent genes in tumor recurrence and chemosensitization. This goal will be achieved by utilizing a novel 2nd generation CRISPR-Cas9 gene activation system. A catalytically dead Cas9 is fused to histone acetyltransferase p300. This fusion product in combination with a sgRNA delivers the histone acetyltransferase(p300) to a promoter that acetylates nearby histones, leading to selective gene expression. We anticipate that our screen will identify novel genes that may be accessible with HDAC therapies to help prevent and/or treat tumor recurrence.